Electrical coupling circuits



March 12, 1946.

H. l. REISKIND ET AL' ELECTRICAL COUPLING CIRCUIT 2 Sheets-Sheet l v Filed June 29, 1943 w mi B F AWJ bMm 0 2&-% Z M L 3 M L HA m w 2 1 y 3 W 4 ZLNQUQQEWJQPF ATTORNEY.

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H. REISKIND, ET AL 2,395,531 I ELECTRICAL COUPLING CIRCUIT Filed June 29, 1945 2 Sheets-Sheet 2 +3 Hun ZEE/s/Z/ND,

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Patented Mar. 12, 1946 ELECTRICAL COUPLING omourrs Hillel I. Reiskind and Alexis Badmaiei'f, Indianapolis, Ind., assignors to Radio Corporation of America, a corporation 01' Delaware Application June 29, 1943, Serial No. 492,682

2 Claims.

This invention relates to electrical current transmission circuits and particularly to electronic coupling circuits for connecting a balanced line either of high or low impedance toa single ended line, thus eliminatin the use of balanced transformers for this purpose.

The usual method of connecting a balanced or triple terminal line to a single ended or two terminal line has been by the use of a balanced transformer which inherently isolates direct current portions of the two circuits and cancels the in phase longitudinal signal in the balanced line. By the use of transformers of different impedances, the output impedance of the balanced line and the input impedance of the single ended line may be readily matched. The present invention is directed to the use of an interconnecting vacuum tube circuit which may be readily substituted for a transformer and which is easily adjustable to provide better results than an expensive transformer. The electronic circuit may be used between balanced lines of either high or low impedance, adjustment being provided for obtaining both amplitude and phase neutralization of longitudinal currents and the proper addition of the transverse signal currents in each half of the balanced line.

Two embodiments of the invention are described hereinafter, one embodiment having a gain higher than unity with a floating cathode and the other embodiment having an over-all gain of unity with a grounded cathode, the latter utilizing the principle of two constant current generators con nected in parallel relation so that their output voltages are additive.

The principal object of the invention, therefore, is to improve the coupling circuits between a balanced line and a single ended line.

Another object of the invention is to provide an electronic interconnecting circuit between a balanced line and a sing e ended line in which the longitudinal voltages are cancelled and the transverse, voltages are transmitted with or without an over-all gain.

A further object of the invention is to provide an interconnecting circuit between a balanced line of either high or low impedance to a single ended line without the use of a transformer.

A further object of the invention is to provide a phase inverter circuit between a three terminal 0 balanced line and a two terminal line.

single ended line.

Although the novel features which are believed to be characteristic of this invention will be pointed out with particularity in the appended claims, the manner of its organization and the mode of its operation will be better understood by referring to the following description read in conjunction with the accompanying drawings forming a part hereof in which:

Fig. 1 is a schematic diagram of the basic circuit of one embodiment of the invention.

Fig. 2 is a schematic diagram of a low, impedance coupling circuit using the basic circuit of Fig. 1.

Fig. 3 is a schematic diagram of a high impedance coupling circuit using the basic circuit of Fig. 1.

Fig. 4 is a schematic diagram of the basic circuit of another embodiment of the invention.

Fig. 5 is a circuit diagram of a low impedance coupling circuit using the basic circuit of Fig. 4, and,

Fig. 6 is a schematic diagramof a high impedance coupling circuit using the basic circuit of Fig. 4. I

Referring now to Fig. 1, the basic circuit is shown between triple input terminals a, b, and 0 adapted to be connected to a balanced line, 0 being the grounded conductor as shown at 5, and output terminals (1 and e adapted to be connected to a single ended'line of relatively high impedance, such as the input to another vacuum tube.

Connected between'terminals a and c is a resistor 6, and connected between terminals b and c is a resistor 1, these resistors being for the purpose of terminating the line and balancing the longitudinal voltages in the balanced line. These longitudinal voltages will not *be impressed on and thereby transmitted by vacuum tube 8, inasmuch as the voltage drop between the control grid and the cathode, due to these voltages, is zero. However, the transverse signal voltages appearing across both resistors! and I will be impressed between the control grid and cathode of the tube 9 and will thus be amplified in the tube 9 and appear across load resistor l0 and be transmitted over coupling condenser l I to the output t rminals d and e. In this type of interconnecting circuit between a balanced line connected at terminals a, b, and c and a single ended line connected at terminal d and e, the full gain of the tube 9 is realized although the direct current in the resistor I must be blocked from the balanced line. Y

Referring now'to Fig. 2 in which is illustrated a specific circuit for coupling a low impedance ended line, such as the high impedance grid circuitof a tube, the same tube 9, load resistor l0, coupling condenser II, and terminating resistors 5 and 1 are used along with voltage divider re sistors I2 and I3 for obtaining a screen grid potential. Elements 5, 1, 9, l0, and II all function in the same manner in Fig. 2 as they did in Fig. 1. In this circuit, however, two 40 mfd. condensers II and ii are shown for blocking from the balanced line the direct current in the circuit of the cathode resistor I6. In addition to resistors I and I for balancing the amplitude of the longitudinal currents in the balanced line, a potentiometer l8 has been provided for simplifying the adjustment of this balance. In addition to the cathode resistor IS, a fixed resistor 20 and a potentiometer resistor 2| are used for balancing the phase of the longitudinal currents in the balanced line. In the coupling circuit for a 500 ohm transmission line, the value of resistors 5 and I was 220 ohms each, and the value of potentiometer resistor I8 was 50 ohms, while an RCA type 6J7 vacuum tube was employed at 9. When the value of resistor I5 is 1200 ohms and .equal to resistors 20 and H combined, and the resistor i is 56,000 ohms, the effective voltage gain of the tube 9 is substantially 56. If the resistor i6 is made 2700 ohms and equal to resistors 20 and 2! with resistor N of 25,000 ohms, the effective voltage gain is substantiallylii.

Referring to Fig. 3 a coupling circuit embody-- ing the principles shown in Fig. 1 is shown for coupling a high impedance balanced line, such as the output of a variable gain amplifier, to a single ended line such as a high impedance grid circuit of a tube. Th input of this coupling circuit is shown connected to the plates 23 and 24 of a pair of vacuum tubes, which may be RCA type 6K7's', the plates being supplied with voltage from the .8 plus terminal over conductor 25 and load resistors 25 and 21, respectively, of 56,000 ohm each. The amplitude balancing resistor 29 may havea value of 250,000 ohms, while the blocking condensers 3| and 32 have a value of .05 mfd. It will be noted that a'condenser 33 of 10 mfd. is provided for the potentiometer resistor 29. the purpose of this condenser being to prevent direct current flow across the contact.v

In a circuit of this type, one of the major difflculties is that the cathode resistor l (see Fig. 2) of the tube 9 should be high to prevent loading of thevariable gain amplifier stage and a high value of this resistor provides too high a grid bias for the tube 9. This difilculty is overcome in Fig. 3 by using only a portion ofthe resistor to supply grid bias. The division is such 2,896,581 500 ohm balanced transmission line to a single.

coupling a high impedance balanced line to a single ended line.

Referring now to the second embodiment of the invention shown in Figs. 4, 5, and 6, Fig. 4 shows the basic circuit. These circuits operate on the principle that the outputs of two constant current generators connecting in parallel will add.

d and e are shown as in Figs. 1, 2, and 3, with terminating resistors 50 and Si being connected across each half of the balanced line. The signal or transverse voltages between terminal a and ground terminal 0 is fed through a resistor 53, while the signal between terminal b and ground terminal c is fed through inverte tube 55 which is a pentode and which is adjusted to unity gain or balance by a cathode resistor 51. This unity gain adjustment might also be accomplished by a grid resistor as shown at 52 in Fig. 5. The output im pedance of tube 55 is high and by paralleling its output voltage appearing across load resistor 59 with the voltage across resistor 53 and feeding these voltage to a high impedance circuit, such as the input of a vacuum tube connected across terminals d and e, the two outputs will add. In

fcuit of Fig. 4 has been used for coupling a low impedance balanced line to a high impedance single ended line, the balancing resistors 50 and SI have a value of 270 ohms, while the resistor 53 has a value of 12,000 ohms and the load resistor 56 has a value of 100,000 ohms. A potential divider consisting of resistors 59 and 60 of 57,000

and 56,000 ohms, respectively, is shown for obtaining a potential for the screen grid of the tube 55. In this circuit the. cathode resistor 51 has a resistance of 2700 ohms. To obtain a unity gain adjustment for the inverter tube 55 which may be an RCA type 6J7, a potentiometer 62 having a resistance of 250,000 ohms is employed, while a condenser 54 of 10 mid. and a condenser 55 of .5 mfd. provides the proper ratio between the resistors 53 and 52. These values are not critical and the balancing of the input resistors and capacitors is not required.' In using circuits of the type shown in Figs. 4, 5, and 6, the capacitance e of the vacuum tube 55.reduces its gain and introthat portion 95 is approximately 1200 ohms and portion 26 is approximately 47,000 ohms, the drop across resistor portion 35 being fed through a 560,000 ohm resistor 38 to the grid of tube 9. A balancing potentiometer resistor 39 of 25,000 ohms is provided in series with a fixed resistor 40 of 47,000 ohms for phase balance of the longitudinal currents. A condensenfl of .5 mfd. is provided as a blocking condenser between the grid of tube 9 and ground. The load resistor III of this circuit is 100,000 ohms, while the voltage divider re istor I2 is 56.000 ohms and the resistor I3 is 47,000 ohms, the tube 9 again being a 6J7. The coupling condenser II has a. value of .1 mfd. Thus; the circuits shown in Figs. 2 and 3 utilize the basic principle shownin Figthe circuit of Fig. 2 coupling a low impedance balanced line" duces some phase shift at the higher frequencies which reduces the cancellation of the in phase or longitudinal frequencies. To compensate for this phase shift, the cathode resistor 57 is shunted by a small adjustable capacitor 55. This effect is not present in the circuits of Figs. 1, 2, and'9, since, in these circuits, all the cancellation occurs in the input circuit, while in the circuits of Figs. 4, 5, and 6, any unbalance existing in the input circuit is balanced out after inversion in the tube 55. A coupling resistor of 100,000 ohms is shown at 59.

, Referring now to Fig. 6 in which the basic principle of Fig. '4 is employed in a coupling circuit between a high impedance balanced line and single ended circuit terminals d and e similar to the terminations of Fig. 3, the same tube 55 is shown witha load resistor 58 of 50,000 ohms. potential divider resistors II and 00 of 47,000

and 56,000 ohms, respectively, and coupling condenser M of .1 mid. along with cathode resistor is? of 2700 ohms shunted by variable condenser and condenser 33 are provided. In this circuit,

in orderto maintain equal voltages and loads on the two tubes of the'variable gain stage, direct current isolation is provided between the upper variable gain tube and the balancing tube by condensers 3i and 32, while potentiometer 28 and potentiometer 62 permit balancing of the variable gain stage and a unity gain adjustment of the tube 55, respectively. Condenser-'66 balances the capacity of tube 55 as in Fig. 5.

In all the circuits of Figs. 4, 5, and 6, the oathode is at ground potential except for the small voltage drop across resistor 51. The signal or transverse voltages from the variable gain amplifier are fed in phase and therefore add, while the longitudinal voltages are cancelled or balanced out after passage through the tube 55.

The variable gain amplifier may be either an expander or a compressor of the type shown in Singer Patent No. 2,255,683, of September 9, 1941, the above coupling circuits being advantageous for coupling the push-pull amplifiers to a two terminal or single ended line such as the grid circuit of a single amplifier stage.

We claim as our invention: y

l. A coupling circuit for connecting the output of a push-pull amplifier with the input of a single stage amplifier comprising-a pair oi terminating the amplitudes oi the longitudinal voltages in the two sections or the output of said push-pull amplifier, a vacuum tube having its input connected to one of said sections and its output connected in parallel with said other section, a potentiometer in the input circuit of said tube for matching the output voltage of said tube with resistors for the output of said push-pull amplifier, a potentiometer connected between the plate circuits or said push-pull amplifier for balancing the voltage in the other section of the output circuit of said push-pull amplifier, and a resistorcondenser circuit connected between the cathode oi said tube and the input of said single stage amplifier for balancing the phase of the output voltage of said tube with the phase of the voltage in the other section of the output circuit of said push-pull amplifier.

2. A coupling circuit for coupling a, three terminal balanced line with a two terminal unbalanced line comprising a pair of terminating re-. sistors, one resistor being connected across each section of said balanced line; a potentiometer connected across the non-grounded terminals of said balanced line to balance the amplitudes of the longitudinal voltages in the two sections of said balanced line, conductors for directly connecting one section of said balanced line with the terminals of said unbalanced line, a vacuum tube having its input connected to said other section or said balanced line and its output connected in parallel with said first mentioned section of said balanced line, a biasing resistor for 

